The present invention is directed generally to an optical device, and more particularly to devices for producing a modulated light beam in response to an applied signal.
Optical communications systems commonly transfer information from a transmitting unit to a receiving unit via an optical fiber or free-space link. The information is typically encoded on an optical carrier by modulating its amplitude, phase and/or frequency.
A modulated optical beam may be generated by changing the junction current of a semiconductor laser or by externally modulating the beam from a continuous wave (CW) laser. Direct current modulation typically changes the frequency and the amplitude of a semiconductor laser output beam. External modulation only changes the amplitude or phase of the output beam, allowing the laser frequency to be tuned independently of the modulation.
External modulators are commonly used in dense wavelength division multiplexed (DWDM) communications systems that combine a number of single-frequency signals to form a multi-channel signal that can be transported by a single optical fiber. To maintain the fidelity of each signal, the frequency of each transmitter is stabilized to an assigned channel frequency. For example, the International Telecommunications Union has established standards for dense wavelength division multiplexed (DWDM) communication signals near 1.5 xcexcm that assign adjacent channels to optical frequencies that are separated by a fixed frequency difference of 50 GHz or 100 GHz. DWDM laser transmitters are typically stabilized to a small fraction of the interchannel spacing (typically 3 GHz) over periods of months or years. This is facilitated by the use of an externally modulated laser transmitter in which the frequency of the output is not affected by modulation.
Laser/modulator systems for use in DWDM and other applications advantageously combine long-term frequency stability and modulation fidelity in a compact package that is long-lived and insensitive to environmental noise. In the ideal case, the device should be also be manufacturable using a low-cost volume manufacturing process. In certain DWDM applications, it is also advantageous to tune the output of an externally modulated laser transmitter from one channel to another in response to an input signal. This may be accomplished by synchronously tuning the laser and modulator.
External modulators are often optically coupled to a semiconductor laser with a lens or alternative optical focusing system. An externally modulated laser assembly must, therefore, provide for the mounting and alignment of these components. In many cases, the mounting structure must also be stable with respect to temperature and vibration over long periods of time. This is difficult to achieve in a low-cost, volume-manufacturable package.
Generally, the present invention relates to a device for the modulation of a light beam, typically a light beam generated by a tunable semiconductor laser. Conventional modulators are often coupled to a semiconductor laser using an optical coupling system. This approach limits the manufacturability, cost and robustness of the laser/modulator assembly.
According to the present invention, the output of a semiconductor laser may be modulated using a coupled waveguide modulator that may be endfire coupled, also referred to as butt coupled, to the laser. The modulator may also be formed on a common substrate with the laser, thereby providing a monolithic laser/modulator system that may manufactured using a high-volume semiconductor manufacturing process. The environmental noise immunity of the system is improved significantly by eliminating the coupling optical assembly.
One particular embodiment of the invention is directed to and optical device that includes a substrate having a coupler region and an output region and a first waveguide on the substrate parallel to an optical axis. A second waveguide is formed proximate the first waveguide, is parallel to the optical axis, and is separated from the first waveguide by a coupling distance in at least the coupler region. The second waveguide is nonparallel to the optical axis and separated from the first waveguide by at least the coupling distance in at least part of the output region.
Another embodiment of the invention is directed to an optical device that includes a laser and a substrate, the substrate having a coupler region and an output region and a first waveguide on the substrate parallel to an optical axis and that is coupled to the laser. A second waveguide is formed proximate the first waveguide and is parallel to the optical axis and separated from the first waveguide by a coupling distance in at least the coupler region. An input to one of the first and second waveguides is optically coupled to receive an output from the laser. The second waveguide is nonparallel to the optical axis and separated from the first waveguide by at least the coupling distance in at least part of the output region.
Another embodiment of the invention is directed to an optical communications system that includes a transmitting unit, a receiving unit and an optical transport system. The transmitting unit generates an optical information signal that is carried to the receiving unit by the optical transport system. At least one of the transmitting unit, the receiving unit and the optical transport system have at least one optical device that includes a substrate having a coupler region and an output region, with a first waveguide on the substrate parallel to an optical axis. A second waveguide is formed proximate the first waveguide, parallel to the optical axis, and separated from the first waveguide by a coupling distance in at least the coupler region. The second waveguide is nonparallel to the optical axis and is separated from the first waveguide by at least the coupling distance in at least part of the output region.
Another embodiment of the invention is directed to an optical communications system that includes a transmitting unit, a receiving unit and an optical transport system. The transmitting unit generates an optical information signal propagating to the receiving unit through the optical transport system. At least one of the transmitting unit, the receiving unit and the optical transport system has at least one optical device that includes a laser and a substrate having a coupler region and an output region. A first waveguide on the substrate is parallel to an optical axis and a second waveguide formed proximate the first waveguide is parallel to the optical axis and is separated from the first waveguide by a coupling distance in at least the coupler region. The second waveguide is nonparallel to the optical axis and is separated from the first waveguide by at least the coupling distance in at least part of the output region.
Another embodiment of the invention is directed to a method for modulating a light beam that includes coupling light to a first waveguide formed on a substrate with a coupler region and an output region, the first waveguide being parallel to an optical axis, and transferring light from the first waveguide to a second waveguide proximate the first waveguide in the coupler region. A current is controlled through the first waveguide and the second waveguide to control an amount of light coupled from the first waveguide to the second waveguide. The coupled light in the second waveguide is directed in a direction that is nonparallel to the optical axis in at least part of the output region.
The above summary of the present invention is not intended to describe each illustrated embodiment or every implementation of the present invention. The figures and the detailed description which follow more particularly exemplify these embodiments.